The present invention concerns a passive maser and a process for controlling a passive maser, wherein a first error signal represents the difference between the carrier frequency of an injected signal and the frequency of the stimulated emission, and is used to adjust the frequency of the injected signal, while a second error signal represents the difference between the resonance frequency of the cavity and the frequency of the injected signal, and is used to adjust the resonance frequency of the cavity. By virtue of its excellent frequency stability, such a maser is particularly suitable for forming a frequency standard.
A maser of this kind is known, in particular from the article by F. L. Wall: Proceeding of P.T.T.I. 1976, pages 369-380. A detailed description of this maser will be given hereinafter so as clearly to show the limitations thereof and to provide for full understanding of the present invention. It will suffice to mention here that the high-frequency signal introduced into the cavity is phase-modulated at two different frequencies: a relatively low frequency which is used for controlling the carrier frequency defined from the base oscillator, and a relatively high frequency which is used for controlling the resonance frequency of the cavity. A maser of this kind operates correctly only insofar as the two control loops or circuits are indeed independent.
The inventors have analyzed the extent to which the two control circuits are actually independent. They found that the cavity resonance frequency control circuit did not interfere with the injected interrogation signal frequency control circuit (also referred to as the oscillator control circuit) but that, on the other hand, the latter circuit which is sensitive to the very narrow line of the stimulated emission, caused a parasite signal to be introduced into the error signal of the circuit for controlling the maser cavity, the parasite signal completely destroying the correct error signal. The inventors have verified this fact experimentally and they have found a theoretical explanation for this phenomenon.
The object of the present invention is to prevent the error signal of the cavity control circuit being interfered with by the parasite signal of the oscillator control circuit.